Thanks for the replies @ChubbyRain, @Brass-Man and @KingLeovold. Pre restriction, I would always play a 2 - 2 split of Flusterstorm and Pyroblast in Mentor decks but have found Flusterstorm to be less good since the restriction.
Last week I spent an evening trying out Scabs' Esper Mentor deck. His list played 3 Flusterstorm and I was interested to see how it would perform against Jeskai Mentor. We played for about 5 hours and it didn't go well for Esper. One of the things I missed most was Pyroblast, especially because my friend was playing 2 Jace, TMS which was often quite tough to deal with.
As Matt said, I have also found that Flusterstorm is quite strong in Thirst for Knowledge & Paradoxical Outcome decks.
Thanks again for your replies!